Multicomponent boron-containing carbide coatings (i.e., (Zr,Ti)C_xB_y) on a C/C composite show good ablation resistance. However, the high-temperature oxidation behavior of this new type of boron-containing (Zr,Ti)C_xB_y solid solution ceramics has not been clarified yet. The present work fabricated (Zr,Ti)C_xB_y solid solution block ceramics by spark plasma sintering, and their oxidation behavior at 1600 ℃ in air (N₂–20-vol% O₂) was investigated for the first time. The effects of boron on the oxidation resistance of (Zr,Ti)C_xB_y ceramics were examined. The results indicate that the (Zr,Ti)C_xB_y ceramics display good oxidation resistance with the parabolic rate law describing the oxidation process. After the trace solution of boron (0.5 wt%) into (Zr,Ti)C_x, the oxidation resistance of carbide ceramics is significantly enhanced, leading to a decrease of 30% in the oxidation rate constant. The formed oxide scale in the (Zr,Ti)C_xB_y ceramics is dense, and the interlayer shows stronger ability to inhibit inward diffusion of oxygen. In addition, the introduction of boron leads to more negative binding energy of (Zr,Ti)C_xB_y and improves the oxidation resistance of carbides.

Multicomponent ultra-high temperature ceramics (UHTCs) are promising candidates for thermal protection materials (TPMs) used in aerospace field. However, finding out desirable compositions from an enormous number of possible compositions remains challenging. Here, through elucidating the role of preferential oxidation in ablation behavior of multicomponent UHTCs via the thermodynamic analysis and experimental verification, the correlation between the composition and ablation performance of multicomponent UHTCs was revealed from the aspect of thermodynamics. We found that the metal components in UHTCs can be thermodynamically divided into preferentially oxidized component (denoted as M_P), which builds up a skeleton in oxide layer, and laggingly oxidized component (denoted as M_L), which fills the oxide skeleton. Meanwhile, a thermodynamically driven gradient in the concentration of M_P and M_L forms in the oxide layer. Based on these findings, a strategy for pre-evaluating the ablation performance of multicomponent UHTCs was developed, which provides a preliminary basis for the composition design of multicomponent UHTCs.

Multi-component solid solutions with non-stoichiometric compositions are characteristics of ultra-high temperature carbides as promising materials for hypersonic vehicles. However, for group IV transition-metal carbides, the oxidation behavior of multi-component non-stoichiometric (Zr,Hf,Ti)C_x carbide solid solution has not been clarified yet. The present work fabricated four kinds of (Zr,Hf,Ti)C_x carbide solid solution powders by free-pressureless spark plasma sintering to investigate the oxidation behavior of (Zr,Hf,Ti)C_x in air. The effects of metallic atom composition on oxidation resistance were examined. The results indicate that the oxidation kinetics of (Zr,Hf,Ti)C_x are composition dependent. A high Hf content in (Zr,Hf,Ti)C_x was beneficial to form an amorphous Zr-Hf-Ti-C-O oxycarbide layer as an oxygen barrier to enhance the initial oxidation resistance. Meanwhile, an equiatomic ratio of metallic atoms reduced the growth rate of (Zr,Hf,Ti)O₂ oxide, increasing its phase stability at high temperatures, which improved the oxidation activation energy of (Zr,Hf,Ti)C_x.